Accepting, and stopping

If the answer is no, take it gracefully and immediately — understood, thank you for checking — and do not let it sour the start. A no to a reasonable, well-made request is information about the organisation's constraints, not about what they think of you.

Get the final terms in writing before you resign from anything or decline anything else: salary, title, start date, probation length, and anything you were told verbally. Then close your other conversations properly. How you handle the last five days before starting is the first impression that actually sticks.